Concise syntheses of arabinogalactans with β-(1→6)-linked galactopyranose backbones and α-(1→3)- and α-(1→2)-linked arabinofuranose side chains Original Research Article

4-Methoxyphenyl glycosides of 2,3″-bis-α-l-arabinofuranosyl branched β-d-(1→6)-linked galactopyranosyl tetraose (16), 3′,2″″-bis-α-l-arabinofuranosyl branched β-d-(1→6)-linked galactopyranosyl hexaose (27), and a twentyose (42) consisting of β-(1→6)-linked d-galactopyranosyl pentadecaoligosaccharide backbone with α-l-arabinofuranosyl side chains alternately attached at C-2 and C-3 of the middle galactose residue of each consecutive β-(1→6)-linked galactotriose unit of the backbone, were synthesized with isopropyl 3-O-allyl-2,4-di-O-benzoyl-1-thio-β-d-galactopyranoside (6), 2,3,4,6-tetra-O-benzoyl-α-d-galactopyranosyl trichloroacetimidate (7), 2,3,5-tri-O-benzoyl-α-l-arabinofuranosyl trichloroacetimidate (12), 6-O-acetyl-2,3,4-tri-O-benzoyl-α-d-galactopyranosyl trichloroacetimidate (17), 4-methoxyphenyl 2,3,4-tri-O-benzoyl-β-d-galactopyranoside (19), and 2,6-di-O-acetyl-3,4-di-O-benzoyl-α-d-galactopyranosyl trichloroacetimidate (28) as the key synthons. Condensation of 6 with 7 gave the disaccharide donor 8, and subsequent condensation of 8 with 4-methoxyphenyl 2,3,4-tri-O-benzoyl-β-d-galactopyranosyl-(1→6)-2-O-acetyl-3,4-di-O-benzoyl-β-d-galactopyranoside (9) followed by selective deacetylation afforded the tetrasaccharide acceptor 11. Coupling of 11 with 12 gave the pentasaccharide 13, its deallylation followed by coupling with 12, and debenzoylation gave the hexasaccharide 16 with β-(1→6)-linked galactopyranose backbone and 2- and 3″-linked α-l-arabinofuranose side chains. The octasaccharide 27 was similarly synthesized, while the twentyoside 42 was synthesized with tetrasaccharides 33 or 24 as the donors and 23, 36, 38, and 40 as the acceptors by consecutive couplings followed by deacylation.